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Building the biggest galaxiesBY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 03, 2008

Astronomers have caught multiple massive galaxies in the act of merging about 4 billion years ago, supporting the favoured ‘hierarchical’ theory of how galaxies form.

Composite colour image of the brightest galaxies in four groups located about 4 billion light years away. Image: ESO.

The hierarchical model describes a step by step process in which small galaxies merge to build larger ones, meaning that massive galaxies grow through many merging events in their lifetime. However, this type of growth for the brightest galaxy clusters has been debated for nearly 30 years, since uncertainties surround the timing of when the most massive galaxies acquired the majority of their stars, and the stage at which the galaxies’ growth is terminated.

“Mainly, each time galaxies merge, we thought the merging would generate new stars but this is not what we see in these very big galaxies; the stars here are all very old,” team leader Kim-Vy Tran from the University of Zurich, Switzerland tells Astronomy Now. “The realisation is that in these mergers, the fuel needed to make new stars has been removed through other processes, so when the galaxies merge, they cannot produce new stars.”

The main alternative theory to the hierarchal model is that these brightest galaxies obtained all their mass a significantly long time ago – more than eight billion years ago – and haven't seen any action since then. To help answer some of the outstanding mysteries of cluster formation, Tran and her colleagues used the Hubble Space Telescope and the ESO’s ground based Very Large Telescope (VLT) to study massive galaxies in clusters located four billion light years away, in an unusual system made of four galaxy groups that are destined to merge into a single cluster.

"Whether the brightest galaxies in clusters grew substantially in the last few billion years is intensely debated,” says Tran. “Our observations show that in this time, these galaxies have increased their mass by 50 percent."

The four brightest galaxy groups ordered in increasing stellar mass, i.e. a rough time sequence. The brightest galaxies in the first two frames have bright companions (labelled with a + in the top row), while the brightest galaxy in frame 3 also has a double nucleus (see inset in bottom row), showing they are in the process of merging. Image courtesy Kim-Vy Tran.

The astronomers identified a total of 198 galaxies belonging to the four clusters and found that the brightest galaxies in each group contain between 100 and 1000 billion of stars, a property that makes them comparable to the most massive galaxies belonging to clusters. The most surprising result, however, was that in three of the four groups, the brightest galaxy was seen to also have a bright companion galaxy, which, says Tran, is evidence that these galaxy pairs are merging systems.

“The companion galaxies mean that the brightest galaxies do continue to grow even as ‘recently’ as 4 billion years ago and that this growth spurt adds a substantial amount of mass,” she says, “and the interesting point is the companion galaxies also are made of old stars, so we must conclude that the recent merging did not produce a new generation of stars. Most of the stars in these galaxies were born at least 7 billion years ago."

The brightest galaxy in each group can be ordered in to a time sequence of increasing stellar mass, due to the most recent episodes of galactic cannibalism. This discovery provides unique and powerful validation of hierarchical formation as manifested in both galaxy and cluster assembly.